Regional agencies that adopted a plan last year to guide Bay Area land use and transportation through 2040 have agreed with environmentalists to study and explain how they would promote public transit and limit greenhouse gases while building more highways.

The agreement settled a lawsuit by Communities for a Better Environment and the Sierra Club, who argued that Plan Bay Area would increase climate-changing greenhouse gas emissions and displace low-income communities.

The plan also is under a separate legal attack by pro-development forces who claim it is heavy-handed and unnecessary. That lawsuit is before an Alameda County Superior Court judge.

Plan Bay Area, approved in July by the Association of Bay Area Governments and the Metropolitan Transportation Commission, sets guideposts for a 27-year period in which the nine-county population is projected to increase from 7 million to nearly 9 million.

It is not legally binding, but designates areas eligible for state funding to encourage housing and jobs in pedestrian-friendly neighborhoods near transit lines to keep development compact, reduce vehicle use and preserve open space. The agencies have approved 170 "priority development areas," 100 acres or larger, nominated by local governments.

The plan also includes the regional road and transit framework the Metropolitan Transportation Commission updates every four years. But the environmental groups say the commission favors too much expansion of highways - many of which would add car-pool toll lanes - and too little new public transit.

Plan Bay Area is "inconsistent with the agencies' obligations to get people to take public transit," attorney Irene Gutierrez of Earthjustice, a nonprofit representing the environmentalists, said Friday. She argued that the plan would not reduce greenhouse gases on its own and relied instead on California's new standards for vehicle fuel efficiency and low-carbon fuels.

Ezra Rapport, executive director of the Association of Bay Area Governments, replied that the projected population growth makes overall increases in greenhouse gas emissions virtually inevitable, despite the state fuel laws and efforts to reduce urban sprawl.

"Our job is to work with local governments to get compact development, reduce (vehicle) trips and have more amenities and services nearby," he said.

The settlement, reached Thursday, requires the agencies to issue a more detailed analysis of the impact of their development and road-building plans on greenhouse gases when the plan is updated in 2017.

It also calls for them to explain why they granted priority-development status to some areas, like Treasure Island and the Alameda Naval Air Station, that have little access to public transit. Another provision requires more study of projected increases in freight shipments by trucks and ships and the accompanying pollution.

The other lawsuit against Plan Bay Area was filed by the conservative Pacific Legal Foundation on behalf of a group called Bay Area Citizens. They contend the plan would illegally require high-density development - "stack and pack," as the foundation described it - and had failed to consider less-restrictive alternatives.

Rapport described the suit as "fear-mongering" and said the plan consisted of guidelines and incentives, not mandates. Superior Court Judge Evelio Grillo has tentatively ruled in favor of the plan but has not yet issued a final decision.